1. Field of the Invention
This invention relates to a method and a system for bit-rate compression of digital data transmitted between a television transmitter and a television receiver.
2. Description of the Prior Art
In digital television, the luminance and color-difference signals are digitized in eight bits per image point with a structure and a sampling frequency which depend on the problem to be solved. These data are usually transmitted in real time with a very high raw-data bit rate which is higher than 140 megabits per second, thus making it necessary to reduce this bit rate in order to permit the use of existing transmission media by utilizing the high redundancy of these data.
Different methods and devices for bit-rate reduction are already known. Among these, the methods of coding by modulation of differential coded pulses are particularly attractive by reason of their simplicity of application. Known methods of differential coding consist in coding the difference between the value of a sample of the signal to be transmitted and an estimation, or prediction, computed from the values of the preceding samples already coded, this difference being quantized by a quantizer having n quantization levels. With each level i is associated a code C.sub.i which is transmitted over the line or transmission channel. The received code is converted to its real value which is subsequently added to a prediction value computed by the receiver in order to reconstruct the signal. By means of a negative-feedback loop, a prediction can be made at the transmitter and is identical with the prediction formulated at the receiver.
The systems which make use of these methods are of particular interest when the transmitted data are constituted by binary code words of fixed length and when spatial coding is employed, that is, coding which takes into account only states of points which are geographically adjacent to each point to be coded and belong to the same television frame as the point to be coded. In fact, the use of fixed-length code words eliminates the problems of management of the buffer memories which are necessary for adapting a variable bit rate of the source to the fixed bit rate of the channel which connects the transmitter to the receiver, and spatial coding makes it possible to avoid systematic use of an image memory.
However, these systems cannot be employed for television image transmission when the bit rates fall below four bits per transmitted image point since, below this value, the quality of the reconstructed image at the receiver is no longer acceptable. Furthermore, coding which consists for example of three bits per point entails the use of spaced quantization levels which make it difficult to reproduce the image at the receiver, both in regard to the image points contained in the uniform zones or uniform areas and in regard to the image points contained in the contour areas (zones) or highly textured areas. It is found in practice that, in the uniform areas of the image, slight variations in luminance are directly observed by the eye. In order to overcome this defect, it is preferable to quantize the luminance signal of the uniform areas by means of quantizers having closely spaced levels in order to prevent excessive amplification of small luminance variations which might otherwise cause false contours to appear in the vicinity of the zero prediction error. On the other hand, quantization by means of spaced reconstruction levels appears to be better suited to coding of the points located in the contour areas. However, in the second case just mentioned, the spacing between two levels cannot exceed a predetermined limit since the contours which appear beyond this limit are reproduced in the form of stair-steps.
In order to solve these difficulties, one known method consists in changing-over the quantizers to two different quantization characteristics as a function of the local appearance of the image point to be transmitted. For example, in the case of points located in uniform areas of the image, coding is performed by means of a quantizer having closely-spaced reconstruction levels in the vicinity of the zero prediction error. In the case of points located in contour areas or textured areas of the image, a quantizer having high reconstruction levels is employed. However, this method again presents the problem of constructional design of receiving devices which have to identify the instants of any change in quantization and/or prediction characteristics carried out by the transmitter.
If a changeover of the quantization characteristics takes place on points which are not known at the receiver and constitute a non-causal neighborhood for the image points to be decoded, the transmitter must provide the receiver with indications of changes in quantization and/or prediction characteristics. In this case, even if a fixed-length code is employed for coding the quantized prediction errors, the bit rate of each image line is variable and this gives rise to complex problems of management of buffer memories in order to adapt the variable bit rate of the transmitter to the fixed bit rate of the transmission channel.
On the other hand, if the indication relating to a change in quantization and/or prediction characteristics is not transmitted to the receiver, the change in characteristic is obtained in an identical manner at the transmitter and at the receiver from tests carried out on image points which are already known at the receiver and constitute the causal neighborhood of the point to be decoded. Under these conditions, if a fixed-length code is employed, the bit rate of each image line is constant since the only information to be transmitted is the value of the quantized prediction error. However, a problem is presented when, in certain cases, a causal neighborhood does not alone suffice to find the best characteristic of the quantizer or predictors to be employed. This is a particularly crucial problem when the points of the image to be quantized are located within a transition area between a uniform area and a contour area of the image and when this uniform area moves within the image.